\psset{unit=0.4}
%\psset{lightsrc=30 -10 10,linewidth=0.5\pslinewidth}
\psset{viewpoint=50 30 30 rtp2xyz,Decran=50}
\begin{pspicture}(-7,-4)(7,12)
%\psSolid[object=grille,base=-4 4 -4 4,action=draw]
\psSurface[fillcolor=blue!50,ngrid=.25 .25,incolor=yellow,algebraic,linewidth=0.25\pslinewidth,linecolor=gray](-4,-4)(4,4){(x^2+y^2)/4}
\axesIIID(1,1,3)(5,5,7)
\psPoint(3,3,4.5){p}
\psPoint(3,0,0){px}
\psPoint(0,3,0){py}
\psPoint(0,0,3){pz}
\psPoint(3,3,0){pxy}
\psdots[linecolor=red](p)
\psline[linestyle=dashed,linecolor=gray](px)(pxy)
\psline[linestyle=dashed,linecolor=gray](py)(pxy)
\psline[linestyle=dashed,linecolor=gray](p)(pxy)
\psline[linestyle=dashed,linecolor=gray](p)(pz)
\uput[r](p){$(x_0,y_0,z_0)$}
\uput[r](px){$x_0$}
\uput[l](py){$y_0$}
\uput[r](pz){$z_0$}
\end{pspicture}